Improve internal Types

[oscard0m]

From your explanation seems like we should invest some more time on better typing this

The types for this PR are correct it's our internal types that could use improvement, and definitely worth investing improving. However, the gains vs amount of work that'd be involved would be pretty low, so I'd not say it's a super high priority (in particular because of the State type - i.e. we mark eventEmitter as optional, but don't actually check for its existence in a few places; our implementation + test suite ensure it's not undefined at runtime, but the types are not representing that properly).

We can create an issue of it (maintenance label) so at least we keep track of it. Thumbs Up if you are ok with it.

Originally posted by @oscard0m in #422 (comment)